As the demand for cheaper, faster, lower power consuming microprocessors increases, so must the device packing density of the integrated circuit (IC). Very Large Scale Integration (VLSI) techniques have continually evolved to meet the increasing demand. All aspects of the IC must be scaled down to fully minimize the dimensions of the circuit. In addition to minimizing transistor dimensions, one must minimize the dimensions of the field regions (or isolation regions) which serve to physically and electrically isolate one semiconductor device from an adjacent semiconductor device on a semiconductor substrate so that each device can operate independently of the other.
In general, the number of transistors which can be built on a silicon substrate is limited only by the size of the transistors and the available surface area of the silicon substrate. Transistors can only be built in active regions of a silicon substrate while isolation regions of the substrate are dedicated to separating active regions from one another. Therefore, to maximize the number of transistors on the surface of a silicon substrate, it is necessary to maximize the available active surface area of the substrate. The active surface area is maximized by, in turn, minimizing the isolation regions of the silicon substrate. In order to fully minimize an isolation region, the width of the isolation region should approach the minimum width printable by a given photolithographic technology.
One technology which has been developed to form such isolation regions is known as trench technology. A trench isolation structure is formed in a silicon substrate by etching a trench region into the substrate and subsequently refilling this trench with some type of trench fill material. Thereafter active regions adjacent to the trench isolation structure are available for conventional semiconductor processing to form transistors on the semiconductor device.
The material used to fill the trench formed in the semiconductor substrate plays an important roll in the robustness and isolation quality of the trench isolation structure. Typically the trench is filled with a dielectric material such as, for example, a silicon dioxide (oxide). One method of forming an oxide within a trench is by thermal chemical vapor deposition (ThCVD) using ozone.
A ThCVD process is a process by which thermal energy is used to excite reactant gasses in a deposition chamber to form a desired film to be deposited on the surface of a substrate. Because of its highly reactive nature, practitioners have used ozone (O.sub.3) as one of the reactants in a ThCVD process to provide the oxygen necessary to form the oxide film at low temperatures within the range of approximately 300.degree. C. to 500.degree. C. It has been found that the quality of ThCVD oxide films formed using ozone as a reactant gas is highly dependent on the nature of the underlying substrate upon which the oxide is deposited.
For example, it has been found that a high quality ThCVD oxide layer formed using ozone requires nitrogen preconditioning of an underlying oxide film before depositing the ThCVD oxide layer using ozone. During nitrogen preconditioning, an oxide film is subjected to a nitrogen plasma. This nitrogen plasma transforms the top few monolayers of oxide into a nitrogen-rich oxide surface. In doing so, the quality of the surface of this first oxide layer is modified so as to improve the ability of the subsequently deposited oxide layer to stick to the surface of the first oxide layer. As a result, the quality of the ThCVD oxide layer formed using ozone as a source gas is vastly improved.
Unfortunately, ThCVD films exhibit poor across wafer and wafer to wafer film thickness uniformity. Such non-uniformity causes manufacturing problems including, for example, problems associated with chemical mechanical polishing (CMP) of these wafers. Unless the film to be polished by a CMP process is uniform, the danger exists that the CMP process will entirely etch through the film, causing damage to underlying structures. Therefore, the non-uniformity of ThCVD films makes these wafers susceptible to damage by CMP.
One solution to the non-uniformity problem exhibited by ThCVD films is to employ a plasma enhanced chemical vapor deposition (PECVD) process to deposit an oxide layer in the trench. In a PECVD process, electromagnetic energy (RF energy) is used to excite the reactant gases in a deposition chamber to form the desired material deposited on the surface of the substrate. PECVD oxide layers are formed using oxygen (O.sub.2) as one of the reactant gases to provide the oxygen necessary to form the oxide layer. It is well known in the field that the film quality of PECVD oxide layers is not dependent on the nature of the underlying substrate upon which the PECVD oxide is deposited. The plasma energy in a PECVD system more than adequately ensures that the species to be deposited on a substrate are energetic enough to be adsorbed on the surface of the underlying layer with a high sticking coefficient. Consequently, it is well known that preconditioning of the underlying layer upon which a PECVD oxide will be deposited is neither required nor desired.
Low pressure CVD (LPCVD) layers are formed using tetraethylorthosilicate (TEOS) alone or in addition to oxygen (O.sub.2) as a reactant gas to form the oxide layer. However, instead of RF energy to excite the source gases, LPCVD processes typically employ thermal energy by way of high temperatures in the deposition chamber. Because ozone is not used as the reactant gas in a LPCVD process, it is well known that, as with PECVD oxide layers, preconditioning of the underlying layer upon which a LPCVD oxide will be deposited is neither required nor desired.
It has been found in certain processing environments that semiconductor substrates containing trenches filled with oxide films are prone to crystal defects. One such defect is known as a dislocation. A dislocation in a silicon crystal structure can significantly degrade the performance of the semiconductor device by for example, increasing p-n junction leakage and reducing the breakdown voltage. These problems can lead to poor yield and reliability of semiconductor devices.
What is desired is a trench isolation structure which reduces the occurrence of crystal defects yet provides good film thickness uniformity.